This invention relates to a catalytic technique for cracking heavy petroleum stocks and converting olefin gas to valuable ethers. In particular, it provides a continuous integrated process for etherifying olefinic light gas byproduct of FCC cracking to produce C.sub.5.sup.+ ethers. Isobutylene and/or isoamylene containing streams, byproducts of petroleum cracking in a fluidized catalytic cracking (FCC) unit, may be upgraded to ethers by contact with a solid acid catalyst, such as crystalline medium pore siliceous zeolite catalyst.
Recently it has been found that ethers can be produced from FCC iso-olefins by catalytic conversion in a fluidized bed of solid medium pore acid zeolite catalyst. Such a fluidized bed operation typically requires oxidative regeneration of coked catalyst to restore zeolite acidity for further use, while withdrawing spent catalyst and adding fresh acid zeolite to maintain the desired average catalyst activity in the bed. This technique is particularly useful for upgrading FCC light olefinic gas, which usually contains significant amounts of C.sub.4 olefins, including isobutene.
Economic benefits and increased product quality can be achieved by integrating the FCC and etherification units in a novel manner. It is the primary object of this invention to eliminate the etherification catalyst regeneration system which results in significant process investment saving and improved process safety. Another object of this invention is to eliminate the etherification catalyst regeneration which results in significant process investment/operating cost saving. Another object of the present invention is to further extend the usefulness of the medium pore acid zeolite catalyst used in the ether reaction by withdrawing a portion of partially deactivated and coked zeolite catalyst and admixing the withdrawn portion with cracking catalyst in a primary FCC reactor stage. The catalyst withdrawn from the etherification unit operations can be sent directly to the FCC reactor or regenerator; however, it is also feasible to employ the catalyst in other intermediate unit operations, such as olefin upgrading. Prior efforts to increase the octane rating of FCC gasoline by addition of zeolites having a ZSM-5 structure to large pore cracking catalysts have resulted in a small decrease in gasoline yield, increase in gasoline quality, and increase in light olefin byproduct.
Recent efforts have been made in the field of gasoline blending to increase gasoline octane performance without the addition of deleterious components such as tetraethyl lead and benzene. It has been found that lower molecular weight unsymmetrical ethers, such as methyl t-butyl ether (MTBE) and t-amyl methyl ether (TAME) can be added to C.sub.5 -C.sub.10 hydrocarbon-containing gasoline products. Conventional etherification processing uses as catalyst a macroreticular cation exchange resin in the hydrogen form. An example of such a catalyst is "Amberlyst 15". A resin catalyst gives a high conversion rate but is unstable at elevated temperatures (above about 90.degree. C.). When overheated, the resin catalyst releases sulfonic and sulfuric acids. In addition leaching of acid substances from the resin catalyst even at normal operating temperatures causes a reverse reaction--decomposition of ether products to starting materials--to occur upon distillation of ether product. Overall yield is thereby significantly decreased (see U.S. Pat. No. 4,182,913 to Takesono et al).
Etherification reactions conducted over a resin catalyst such as "Amberlyst 15" are usually conducted in the liquid phase below a temperature of about 90.degree. C. and at a pressure of about 150-200 psig. Equilibrium is more favorable at lower temperatures but the reaction rate decreases significantly. Also excess methanol appears to be required to achieve acceptable selectivity. Some recent efforts in the field of etherification reactions have focused on the use of acid medium-pore zeolite catalyst for highly selective conversion of iso-olefin and alcohol starting materials. Examples of such zeolite catalysts are ZSM-4, ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-50, MCM-22 and zeolite Beta, as disclosed in copending U.S. patent application Ser. No. 07/495,667 (Harandi et al/U.S. Pat. No. 5,015,782). Due to lower acidity as compared to resin catalysts, the zeolites need to be employed at higher reaction temperature to achieve conversion rates substantially equivalent to resin catalysts. These solid acid catalyst particles are much more thermally stable than resin catalyst, are less sensitive to methanol-to-isobutene ratio, give no acid effluent, and are easily and quickly regenerated (see Chu et al, "Preparation of Methyl tert-Butyl Ether (MTBE) over Zeolite Catalysts", Industrial Engineering and Chemical Research).
Developments in zeolite catalysis and hydrocarbon conversion processes have created interest in utilizing olefinic feedstocks for producing C.sub.4 -C.sub.5 tertiary olefins, gasoline, etc. In addition to basic chemical reactions promoted by zeolite catalysts having a ZSM-5 structure, a number of discoveries have contributed to the development of new industrial processes for improved FCC operation to enhance iso-olefin production.
It is an object of the present invention to provide a process and apparatus for continuous operation in preparation of alkyl ethers from lower alkanol and C.sub.4 olefin with a conventional acid resin catalyst whereby the resin catalyst is protected from impurities such as nitrogen compounds, metals, and coke. It is a further object to use catalyst from such etherification unit operations as makeup for FCC units.